1. Field of the Invention
The present invention relates to a reflection-type liquid crystal display device for use in low power consumption devices such as a portable terminal and the like.
2. Description of the Related Art
A reflection-type liquid crystal display device performs display by taking environmental light such as interior illumination or sunlight thereinto and reflecting it by a reflector to the observer side. This reflection-type liquid crystal display device has low power consumption because of no need for a backlight and is in wide use for a portable terminal, PDA, and so on. display through use of environmental light as described above, but tends to be relatively inferior in display characteristics such as a contrast ratio, viewing angle, and so on. Hence, the following measures are considered to improve the display characteristics.
(Measure 1)
A reflection-type liquid crystal display device is proposed which uses a vertical alignment-type liquid crystal display panel since it can realize a high contrast ratio and high reflection intensity. In these devices, vertically aligned liquid crystal molecules tilt in a O-degree azimuth where voltage is applied. As shown in FIG. 45A, a quarter-wave (λ/4) plate 101 is arranged here above a front surface of the vertical alignment-type liquid crystal display panel, and black display is performed where the application voltage is turned off.
Regarding this λ/4 plate 101, such a technique is also proposed that the plate is composed of two plates, that is, a half-wave (λ/2) plate 103 and a λ/4 plate 104 as shown in FIG. 45B to reduce wavelength dispersion of birefringence thereof. Note that only a reflecting electrode, a liquid crystal layer, and a transparent electrode are shown representing a liquid crystal panel in FIGS. 45A and B for convenience.
A liquid crystal molecule substantially vertically stands in a state where no voltage is applied to enable realization of ideal black display. Besides, in a state where voltage is applied, when a practical retardation of the liquid crystal layer becomes λ/4, white display is performed. In this reflection-type liquid crystal display device, it is effective to provide a negative retardation plate 102 between the liquid crystal panel and the λ/4 plate 101 in order to improve viewing angle characteristics. The existence of the negative retardation film compensates the retardation of the liquid crystal layer.
(Measure 2)
In the case in which environmental light is reflected by a reflector to the observer side, when the reflector is composed of a smooth mirror surface, display becomes bright in a regular reflection region and becomes dark in other regions, leading to great dependence on viewing angle, and the display in the regular reflection region may have metallic luster. Hence, such there is a known technique in which projections and depressions in dot-like plane shapes are formed on the surface of a reflector to disperse reflection light, thereby realizing display with less dependence on viewing angle and no metallic luster (see Patent Document 1).
Further, a twisted nematic (TN) mode using one polarizing plate (see Patent Document 2) is proposed as a display mode. TN mode is such a mode that liquid crystal having a positive dielectric constant anisotropy is twist-aligned in a horizontal direction, in which incident environmental light is converted into linearly polarized light by a polarizing plate for white display where no voltage is applied by rotating its polarization azimuth 180° by a retardation plate having a retardation of about a quarter of the visible light wavelength and a liquid crystal layer, and for black display where voltage is applied by rotating the polarization azimuth 90° by the retardation plate having a retardation of about a quarter of the visible light wavelength.
(Patent Document 1)
Japanese Patent Application Laid-Open No. Hei 6-175126
(Patent Document 2)
Japanese Patent Application Laid-Open No. Hei 6-11711
(Patent Document 3)
Japanese Patent Application Laid-Open No. Hei 11-311784
(Patent Document 4)
Japanese Patent Application Laid-Open No. Sho 60-256121
(Patent Document 5)
Japanese Patent Application Laid-Open No. Sho 62-210423
(Patent Document 6)
United Kingdom Patent Specification No. 1,462,978
(Patent Document 7)
Japanese Patent Application Laid-Open No. Hei 10-153802
(Patent Document 8)
Japanese Patent Application Laid-Open No. 2001-4999
(Patent Document 9)
Japanese Patent Application Laid-Open No. 2000-35570
(Patent Document 10)
Japanese Patent Application Laid-Open No. 2000-171799
(Patent Document 11)
Japanese Patent Application Laid-Open No. 2001-154190
(Patent Document 12)
Japanese Patent Application Laid-Open No. Hei 6-337421
(Non-Patent Document 1)
The journal of the Institute of Television Engineers of Japan Vol. 50, No. 8, pp 1091-1095, 1996
However, the above-described measures for improvement of display characteristics have problems as follows:
In the case of the measure 1, production of the negative retardation film requires a special and very advanced technique, such as, stretching of an optical film in two directions, or application of a special optical substance onto a film being a base, and thus the negative retardation plate is expensive.
A technique for easily realizing this negative retardation film is proposed by Tohoku University. This is produced by layering uniaxially stretched films each having a desired negative retardation and an in-plane retardation the same as the negative retardation which are orthogonal. This case, however, needs many films, leading to a problem that the display becomes thick or expensive.
In the case of the measure 2, liquid crystal anchoring to the interface of a substrate still remains without switching after application of voltage in the TN mode, leading to a problem that black luminance floats and thus a sufficient contrast ratio cannot be obtained. The visibility of display is defined by the brightness and contrast ratio, in which display is easy to view even at a low contrast ratio if it is bright, while display needs to have a high contrast ratio if it is dark (see Non-Patent Document 1). The reflection-type liquid crystal display device performs display by taking environmental light thereinto and reflecting it by a reflector to the observer side, and therefore the display is dark in an indoor environment and insufficient in contrast ratio to become hard to view.
As means for improving the contrast ratio, proposed is a technique for compensating black display by substantially matching a slow axis of a retardation plate with an anchoring azimuth of the liquid crystal layer, and decreasing the retardation of the retardation plate by a retardation remaining in the liquid crystal layer (residual retardation) (see Patent Document 3). The technique has, however, problems that a sufficient contrast ratio cannot be obtained because of colored black display due to wavelength dispersion of the liquid crystal layer greater than that of the retardation plate, and that since the retardation of the liquid crystal layer needs to be increased by the decreased retardation for white display, the driving voltage should be increased to obtain the same contrast ratio.